1. FIELD OF THE INVENTION
This invention relates to the optically active ether derivatives represented by the formula (I): ##STR2## (wherein X represents --COO-- or --OCO--; A represents an alkyl or alkoxyl group having 3 to 15 carbon atoms; R represents an alkyl or alkyloxyalkyl group having 1 to 20 carbon atoms; l and m each represents a number of 1 or 2; and * indicates an asymmetric carbon atom), preparation processes therefor, liquid crystal materials containing such ether derivatives as active ingredient, and an optical switching element using said liquid crystal materials as liquid crystal element.
2. DESCRIPTION OF THE PRIOR ART
The optically active ether derivatives represented by the formula (I) are the novel compounds found out for the first time by the present inventors. These compounds are useful as liquid crystal compounds, especially ones showing excellent responsiveness in video display. Further, the compounds of this invention can be worked into liquid crystal compositions that can be utilized as a liquid crystal element for producing an optical switching element.
When the term "liquid crystal compounds" is used in this specification, it means not only those compounds which are per se capable of forming a liquid crystal phase but also the compounds which are useful as a component to blended for a liquid crystal even if they per se cannot be observed as a liquid crystal phase.
Image display devices utilizing liquid crystal are now widely provided for practical application, and particularly, TN (twisted nematic) type display system is popularly employed for such devices.
This system has many advantages such as small power consumption and softness to the eye because of the light-receiving type display panel which itself is not luminous, but on the other hand it has the defect that the response speed in image display is low.
High-speed response is however, especially required in the recent image displays, and many efforts have been made for improving the response characteristics of the liquid crystal compounds. However, the above-mentioned twisted nematic type display system still can not stand comparison with the luminescent type display systems such as light-emitting diode, electroluminescence or plasma displays in response time.
Efforts have been continued for finding out a new display system which is capable of high-speed response while making full use of the advantages of liquid crystal displays which are light-receiving type and low in power consumption, and one result thereof has been the proposal of a display device utilizing the optical switching phenomenon of ferroelectric liquid crystal such as disclosed in Applied Physical Letter, 36, 899 (1980). This ferroelectric liquid crystal was discovered by R. B. Mayer et al (Journal de Physique, 36, L-69 (1975)) and is considered to fall in the same category as chiral smectic C phase (hereinafter abbreviated as Sc* phase) in terms of molecular arrangement. The display devices utilizing such ferroelectric liquid crystal are divided into types: birefringence mode using two pieces of polarizer and a liquid crystal element and gest-host mode using one piece of polarizer and a liquid crystal element mixed with a dichroic dye.
Regarding the liquid crystal compounds used in these display systems, in the case of birefringence mode, a material having a tilt angle of 22.5.degree. is preferably used as such material can give the optimal contrast, while in the case of gest-host mode, a material having a tilt angle as close to 45.degree. as possible is preferably used.
However, there has yet been found no liquid crystal material which shows spontaneous polarization necessary for high-speed response, is chemically stable and also has a high tilt angle and liquid crystal properties in the low temperature region. Use of a liquid crystal material with high tilt angle is also important for allowing free adjustment of tilt angle of the composition comprising such liquid crystal material.
In view of the above, the present inventors have made extensive studies for developing a more useful ferro-electric liquid crystal showing sufficient spontaneous polarization, chemically stable (halogen-free in the molecule) and capable of high-speed response and, as a result, have succeeded in achieving the present invention.